Optical and acoustic (e.g., ultrasound) imaging have each been used as separate modalities for diagnostic imaging. Each of these modalities has limitations owing to the physics involved.
For example, while ultrasonic energy used for imaging can have very good penetration, e.g., it can penetrate through up to 20 centimeters of tissue without much signal attenuation, the available resolution depends upon the spatial pulse length used. For example, one cycle of a pulse of a 5 MHz signal will provide a resolution of about 0.3 millimeters. (This estimate is based on a propagation speed of 1.5 mm/.mu.s; i.e., one cycle=0.2 .mu.s.times.1.5 mm/.mu.s=0.3 mm.) In other words, with a frequency of 5 MHz, one can not detect anything smaller than 0.3 millimeters or differentiate two features with a separation of less than 0.3 millimeters.
Optical energy, on the other hand, offers good resolution but very limited penetration, due to attenuation and scattering of light in tissue. The resolution possible with optical imaging is based on the wavelength of the light. Typical optical imaging wavelengths are in the micrometer range, and therefore resolutions on the order of micrometers are possible. Moreover, since tissues with different metabolisms (such as cancerous tissues) often exhibit different optical absorption characteristics, different optical wavelengths may be advantageously employed for imaging and detection of abnormal tissue.
Thus, optical imaging is hampered by relatively poor penetrability and difflusion, and ultrasound imaging is hampered by its relatively poor spatial and contrast resolution for defining the physiology of images. These modalities may be combined, however, to provide new and improved systems and methods for imaging. Copending U.S. patent application Ser. No. 08/993,165, filed Dec. 18, 1997, entitled "Optoacoustic Contrast Agents and Methods for Their Use," which is hereby incorporated by reference in its entirety, discloses novel optoacoustic contrast agents and methods of imaging using such contrast agents. The present invention was made during the process of designing an imaging system and associated optoacoustic transducer for use, preferably, with the novel contrast agents.